(violateRights (shackle (humiliate userID)))
Or similar.
"Usage of this backdoor will inform the user that they are being surveilled"
I don't think you can execute a National Security Letter gag order against source code.
It's like saying you can shoot somebody because a gun can't be prosecuted for murder.
The key is to make sure that they get access to the backdoor directly.
At the end of the day you can always code loopholes around the law, which will work until they legislate the loopholes away with more legislation. Almost every industry has been doing this since the beginning of government.
One of the best ways to code this into the system is to provide security measures that allow you to prevent what machines can access your account and notify you if machines not your own try to access your account.
"You recently tried to access your account from the IP address X.X.X.X located in Quantico, Virginia. If this was not you, please report this to customer service."
Customer service can then legitimately comply with the gag order by admitting nothing and say they'll look into it.
Right, so you can't prosecute Google if I use gmail to send something that violates a gag order. If Google violate a gag order through their source code they are entirely liable for it.
Gag orders are circumstantial. Code that applies to general situations is not. Backdoors (access) and confidentiality (disclosure) are concepts that are mutually exclusive of one another.